Containerized shipping uses rectangular shipping containers of standard dimensions. The containers are commonly twenty feet or forty feet in length and are transported on container ships, stacked one on top of the other. The loading and unloading of the containers is commonly accomplished using overhead trolley cranes of the type having an overhead track which extends from the wharf or pier out over the container ship. The overhead track is supported by uprights, the uprights being supported on the pier by wheels. The wheels ride along tracks on the pier so the crane can move along the pier in what is called gantry directions. The overhead track and upright supports constitute the trolley support.
A trolley is supported by the overhead track and moves along the overhead track in off shore, that is away from the pier, and on shore directions called trolley directions. The trolley commonly includes a cab, within which the crane driver sits, and a container bridle. (The cab can, however, be mounted in a stationary manner to the trolley support.) The container bridle is raised and lowered by the operator and is used to pick up the containers at their corners during loading and unloading operations. The corners of the containers have special corner castings to which the bridle is secured. When desired to move the bridle over the ship forward or aft, the crane driver moves the trolley support and trolley therewith in the desired gantry direction by movement of the entire crane.
To insure that the containers are properly vertically situated, and to help prevent horizontal shifting between levels of containers, alignment cones are commonly used between stacked containers. After a level of containers has been placed on the container ship, a worker is lifted onto the top surface of the containers and places alignment cones in each of the corner castings. After doing so, the worker leaves the top level of containers and the next level of containers is placed on the container ship using the crane.
To keep the containers from shifting, various types of lashings are used to secure the ends of the containers to the ship. Therefore, space is left between the rows of the containers to permit the lashing of the containers to the ship. It has become apparent that when the lasher is working on the top of several levels of containers, the possibility of falling off the edge of the containers exists. To help prevent such accidents, safety systems have been devised.
One safety system uses nets positioned to basically prevent workers from falling from the containers. Erecting the safety nets is often quite difficult and itself exposes workers to the risk of falling. Another system uses a heavy, stationary structure, typically formed in the fashion of a heavy cage, on the top level of the containers. A variable length safety line extends from the heavy cage to a harness on the worker to permit the worker relative freedom of movement over the entire top level of containers. If the worker falls off the edge of the layer of containers, this will cause the safety line to begin to extend rapidly. This rapid extension is halted by a suitable clutch mechanism, similar to those used on a seat belt, so to keep the worker from falling all the way to the bottom of the ship. This system, however, has problems in that if the safety line is at a full extension, and if the worker falls off the edge, while the safety line may not get any longer, it still may be long enough to allow the worker to strike against the ship structure or containers. Also, the cage must be moved onto and off of the top level of containers between the loading or unloading of each level.